This invention relates to a multiple-phase, brushless DC motor. More particularly, it relates to a motor characterized by relatively low cost as well as reduced cogging and torque ripple effects.
The invention is directed to small motors in applications where smoothness of output, low cost and compactness of size are highly desirable. An example of such an application is a magnetic tape drive for multiple track tapes contained in small cartridges or cassettes. In most motors the torque varies with the angular position of the armature so that under load both the torque and the rotational velocity undergo cyclic variations. If the frequency of this ripple effect is within the pass band of a servo loop controlling the tape velocity or tension, it can upset the operation of the loop.
Also in a multiple-track tape drive with a single head, the head is moved across the tape to position it over selected tracks. Servo signals embedded in the tracks are used by a head-positioning servo loop to maintain the proper head position. Again, the ripple effect, which modulates the servo signals retrieved from the tape, can degrade operation of the positioning system.
Ripple is associated with both of the mechanisms by which an electric motor exerts torque. The first of these results essentially from variations in reluctance in the magnetic paths as a function of armature position. To the extent that there are such variations, the motor will exert a torque tending to rotate the armature to a position of minimum reluctance. In general, in a salient pole motor there are a plurality of such positions. In a motor employing permanent magnets in the armature, this phenomenon causes the armature to assume preferred rest positions in the absence of field excitation and for this reason it is often referred to as "cogging".
Torque is also produced by the interaction between fields generated in the armature and in the field. This torque is proportional to the variation of energy in the magnetic field in the air gap as a function of armature angle. It also is subject to ripple, since it varies with armature position.
The torque due to field interaction is substantially greater than the torque resulting from reluctance variation. Accordingly, when the motor operates under full power, the "torque ripple", due in the interaction torque is predominant. The "cogging ripple" is a substantial factor when the motor operates under reduced power, i.e. when it is decelerating, since the reluctance torque is then an appreciable part of the total torque.
Brushless DC motors are energized in multiple phase fashion, a three-phase arrangement usually being used for simplicity of circuitry. To minimize ripple, the windings are generally connected in a delta arrangement. Moreover, to obtain substantial torque in a compact size, the armature magnets are generally made of relatively expensive rare earth elements. The principle object of the present invention is to provide a brushless DC motor having a small size and characterized by relatively high efficiency and low ripple content in the mechanical output of the motor.